Bi4T3O12 (BTO) is a ferroelectric material that has a typical perovskite layered structure and strong anisotropic properties. The spontaneous polarization, Ps, and the coercive field, Ec, of the crystal along the C-axis are 4 μC/cm2 and 3.5 kV/cm, respectively. The spontaneous polarization and coercive field of the crystal along the a-axis is 50 μC/cm2 and 50 kV/cm, respectively. The dielectric constants of the BTO thin film are 130 and 160 long C-axis and A-axis respectively. Therefore, C-axis BTO is more suitable for FET type of nonvolatile memory applications.
C-axis BTO thin film has been successfully grown on substrates, such as Pt, Bi2SiO5 and Bi2Ti2O7 buffer layer, however, because the processing temperature of BTO thin film is very high, e.g., 700° C. to 800° C., the stability of a bottom electrode on which the BTO film is grown is important in order to integrate the MFM capacitor into devices. A platinum/titanium bottom electrode usually has severe hillock growth and exhibits peeling during high temperature annealing.
U.S. Pat. No. 6,288,420, to Zhang et al., granted Sep. 11, 2001, for Composite iridium-metal-oxygen barrier structure with refractory metal companion barrier, describes use of an iridium-composite electrode which remains conductive, not peeling or forming hillocks, during high temperature annealing processes, even in oxygen.
U.S. Pat. No. 6,236,113, to Zhang et al., granted May 22, 2001, for Iridium composite barrier structure and method for same describes use of an iridium-combination film to prevent oxygen diffusion, and which is resistant to high temperature annealing in oxygen environments.
U.S. Pat. No. 6,190,963, Zhang et al., granted Feb. 20, 2001, for Composite iridium-metal-oxygen barrier structure with refractory metal companion barrier and method for same describes use of an Ir—M—O composite film which is useful in forming an electrode of a ferroelectric capacitor, where M includes a variety of refractory metals. The Ir combination film is resistant to high temperature annealing in oxygen environments.
Kijima et al., Preparation of Bi4Ti3O12 thin films on Si (100) using Bi2SiO5 buffer layer and its electric characterization, J. Appl. Phys. Vol 37, Pt 1, 9B, 5171 (1998), describes preparation of a BTO layer by MOCVD on a silicon substrate and on a buffer layer of Bi2SiO5 on silicon.
Nakamura et al,, Preparation of C-axis-oriented Bi4Ti1O12 thin films by metalorganic chemical vapor deposition, Jpn. J. Phys. Vol. 32, 4086 (1993), describes formation of a BTO thin film on various non-iridium-containing substrates.